The present invention refers to machines for carding textile fibres in which staple fibres are processed so as to produce a web of fibres. During the operation, the fibres are cleaned, stretched out flat, and set parallel to one another in a thin web on the cloth of the carding drum, from which the fibre web is then removed by a rotating cylinder, normally called doffer, provided with a card cloth, and is then passed from a further cylinder, or stripper, to a compacting assembly and a drawing assembly to produce a ribbon of fibres.
In particular, the present invention regards the operation in which the web, which has been removed from the carding drum by the doffer, is then taken up by the stripper and delivered downstream to a controlled assembly for condensing the web into a ribbon and drawing it. This operation is the subject of the copending U.S. patent application No. 09/478,404, dated Jan. 6, 2000 in the name of the present applicant.
For a clearer illustration of the technical aspects and of the problems involved in the taking the web off the carding machine, reference is made to a diagram of the sequence which involves the carding drum, the doffer, the stripper and the compacter, which presented in FIG. 1 in a schematic side view corresponding to the device described in the co-pending patent application, to which the reader is referred for further details as regards the operation and structure of the said device.
Downstream of the main carding drum 1, the doffer of the carding machine is designated by 2; on the latter the fibre web 3 is taken to the stripper 4 and conveyed to the compacting assembly made up of a pair of web-squeezing cylinders 5, 6, which are smooth, rotating and have a substantially horizontal axis; these compact the web 3, the transverse dimension of which still corresponds to that of the carding drum, and pass it on to a subsequent belt conveyor 7 with a vertical working surface, which condenses the web into a ribbon. The compacting assembly works at a speed that is consistent with that of the web 3 coming from the stripper 4. The two web-squeezing cylinders 5 and 6 are driven so that they turn with one and the same linear speed, thus preventing them from sliding and rubbing against one another. The drawn ribbon is then sent on to be collected in a vessel for further processing. Above the stripper 4 is set a device consisting of a rotating brush 8 for cleaning the stripper card cloth. The doffer is contained in the apron 9 made up of containment plates. Underneath the stripper 4 is set a transverse bar 11 for supporting the web 3 that has been stripped by the stripper 4.
Underneath the stripper 4, bar 11 and bottom web-squeezing cylinder 6, is set a suction assembly 12 comprising a funnel-shaped connecting element 14 which is as wide as the generatrix of the doffer 2 and which connects the space 15 beneath the web 3, which extends from the bar 11 to the web-squeezing cylinders 5 and 6, to a suction duct 16 connected to the common aspiration system of the carding machine.
The space 15 is delimited by the scraper blades 17, which are elastically pressed and constantly held so that they adhere and are tangential to the web-squeezing cylinders throughout their width.
A further cleaning treatment of the web 3 upon removal of the latter by means of the stripper 4 is carried out when the web 3 passes from the stripper to the two web-squeezing cylinders 5, 6, which are set immediately downstream of the stripper, by aspirating from the web 3 the impurities, which are also released owing to a the concurrent effect of the web being to a certain extent drawn during its passage through the cylinders, as is described and claimed in the copending patent application cited previously.
FIG. 2 shows a perspective view of a section of the assembly for removing the web from the doffer 2, with the corresponding guards or containment plates all around.
The technical problem regards the web-squeezing rollers 5, 6 and their blades 17. The web-squeezing rollers 5, 6 have the function of conveying and compressing the fibres--including their impurities, for example husks, which are still englobed in them--and may separate the web, causing local cuts, and generate irregularities in the final ribbon which is obtained from condensing of the web, or may even cause tearing of the web and arrest of the machine. The fibres and impurities that undesirably fail to follow the detachment the web 3--which proceeds towards the condensing assembly 7--, and that, instead, remain adherent to the web-squeezing rollers 5 and 6, climb up towards the edge of their scraper blade and tend to work their way into the gap between the inner face of the blade 17 and the surface of the web-squeezing roller, thus adhering even more to the roller.
In order to maintain good operation of the surfaces of the web-squeezing rollers, in the known art the solution is adopted of making, on the said surfaces, spiral-shaped grooves 19 with one or more starts, which, in conjunction with the tangential blades 17, hinder the undesired winding of the fibres. This technical solution makes it possible to reduce winding of the fibres on the rollers but presents the drawback that in the gap between the said blades and the surfaces of the rollers 5, 6 there is an accumulation of material and that on the face of each blade coming into contact with a roller there is an accumulation of material against the edge of the blade and consequent dirtying of the inner face of the latter; this, in turn pushes the blades away from the rollers and is a further cause of the poor cleanliness of the rollers themselves and of a recirculation of fibres in the web. The aforesaid dirtying of the scraper blades 17 and of the web-squeezing rollers 5, 6 calls for periodic manual maintenance operations with the machine not running which must be carried out by qualified staff. The quality of the ribbon produced depends, as a general rule, directly on the frequency of such operations. On the other hand, a high frequency of maintenance operations has an adverse effect on machine output, in so far as the operations require stoppage of the machine and decrease its output factor.